Robert Genn’s latest Painter’s Keys newsletter is discussing copyright online and mentions them awful copyright notices that artists put all over their work.
“Those imprints are called “watermarks,” and while they give the copyright holder a feeling of security, they don’t deter Chinese clone shops from helping themselves. They don’t deter others, either, and it is photographers, particularly, who know all about it. Some pirates think we are living in the last days of copyright and they want to get to the New World. Using low-pixel images will certainly deter someone from making a direct giclee from your image, but no technology will stop somebody making a hand copy of anything you put out there.” Continue Reading Article..
Photographers must have their work stolen more than painters online as they’re much more protective than painters, but it’s a challenge that all artists have to deal with. Most of the copyright lawsuits seem to come from photographers and their estates, so maybe they just don’t like sharing as much as painters 
I don’t think the solution is to place copyright notices all over your work though. It’s something that I dislike with a passion. If the notices are too big or obtrusive I quickly leave the site. I appreciate that the artist is just trying to protect their work, but what can a thief really do with a small, low resolution image on the Internet? A print shop certainly can’t use a pixelated little Internet image to start producing posters to sell online.
The only real solution to the problem is to stay offline, which means missing out on a lot of opportunities. The next best solution is to use a reasonably low resolution image online, but not so low or small an image that it annoys a potential collector. If you must use a watermark or copyright notice, make sure it is hidden in a corner and does not contrast with the art in any way.